1. Field of the Invention
The present invention relates to the improvements of electric arc spraying systems for performing effective thermal spraying.
2. Description of the Related Art
In electric arc spraying, use is made of two consumable metal wires (target wires) each of which is supplied to the corresponding one of two contact chips provided in a spraying gun. In operation, an arc is generated between the target wires, and the heat from the arc melts the tips of the target wires. In accordance with the melting speed, the wires are fed to keep the arc generation. The melted metal is atomized into droplets by compressed gas, and these droplets are injected to the surface being coated.
FIG. 13 shows the configuration of a typical arc spraying system. Specifically, a system power source 1, designed to operate on the commercial power, supplies electric power to a spraying gun 2 under constant-voltage control provided by an inverter control circuit, for example. A compressor 3 generates a jet of compressed gas. The compressed gas from the compressor 3 is supplied via a solenoid valve (not illustrated) in the power source 1, and into the spraying gun 2. Meanwhile, the two target wires are unwound from two wire reels 5a and 5b, respectively, and then sent forward by the “push-side” wire feeders 4a, 4b. These target wires are guided through two guide tubes 6a, 6b to the spray gun 2, which is located away from the wire feeders 4a, 4b. 
The spraying gun 2 is provided with two “pull-side” wire feeders (not illustrated) for moving the target wires, and with two contact chips (not illustrated) to which the target wires are brought for receiving electrical power. The thermal spray voltage and the target wire feeding speed are adjusted by a remote control unit 7.
Referring now to FIG. 2, a recent cylinder block (formed with four bores 8a–8d) used for an automobile engine is made of an aluminum alloy for weight reduction. Each of the bores 8a–8d accommodates a reciprocating piston and is therefore susceptible to abrasion. To protect the bores from such abrasion, an iron sleeve may be inserted into each bore. Alternatively, the inner walls of the bores may be coated with an iron-based material by thermal spraying. This method is more advantageous than the iron sleeve protection since the number of parts is reduced, thereby contributing to the weight and size reduction of the cylinder block.
Thermal spraying to a bore may be performed by inserting a spraying gun into the bore, and then causing the gun to spray in a direction perpendicular to the bore's longitudinal axis. At this time, the gun needs to be rotated about the bore's longitudinal axis so that the spraying is conducted equally to the entire inner wall of the bore that surrounds the gun. However, this thermal spray method is not achievable by the arc spraying system shown in FIG. 13, because the rotation of the spraying gun will unduly twist the guide tubes 6a, 6b since the two push-side wire feeders 4a, 4b are stationary.
In light of the above, plasma spraying or flame spraying is utilized as an alternative to the electric arc spraying because in these methods the spraying gun can be rotated easily. As known in the art, the plasma spraying is a method in which plasma jet is utilized to melt and blast powdery spray material to form a coating on an object. The flame spraying is a method in which flammable gas is burned to melt a spray material and the melted metal is blasted by compressed air onto an object to form a coating. (See JP-A-2004-225101 for example.)
However, the plasma spraying and the flame spraying suffer high running costs due to the use of expensive materials such as the working gas, the combustion gas and the melting substances.